The global positioning system (GPS) is a satellite based radio-navigation system built and operated by the United States Department of Defense. The Russian government operated ‘GLONASS’ and European Union proposed ‘Galileo’ are two other important satellite based navigational systems.
GPS permits a user of the system to determine his or her position on the surface of the earth. The system consists of twenty-four satellites circling the earth at an altitude of about 11,000 miles with a period of about 12 hours. It is possible to have more than twenty-four satellites due to the presence of some spare satellites in the GPS constellation. These satellites are placed in six different orbits such that at any time a minimum of six and a maximum of more than eleven satellites are visible to any user on the surface of the earth except in the polar region. Each satellite transmits an accurate time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it and with signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
Sometimes GPS receivers are required to operate under very weak signal conditions as in foliage or indoors. In the present day practice, the receiver may get “assistance” in the form of additional acquisition aiding messages from a server or base station, or Internet based. But providing this type of assistance requires additional infrastructure and may not be available in all places. Also, the receiver requires additional hardware to receive the aiding messages. Therefore there is a need to develop GPS receivers that operate in “standalone” mode under weak or indoor signal conditions. Further, there is a need as in the case of E911 (Enhanced 911), for fast acquisition of the GPS signals. In addition to the above, the power saving in the receiver is also an important requirement.
Most of the standalone high sensitivity GPS receivers are based on a long non-coherent integration involving squaring loss and thus reducing the possible gain while taking a long time to acquire the satellite signal under weak signal conditions. Regarding the prior art in this field, published U.S. Patent Application 2003/0164795 deals with a type of sleep mode where the clock is kept alive during power off. However, this disclosure does not take into account variations in oscillator temperature and signal Doppler. U.S. Pat. No. 5,893,044 uses an accurate real time clock when the signal is interrupted due to blockage. U.S. Pat. No. 6,320,536 discloses a compensated clock using WAAS (Wide Area Augment System) signals. U.S. Pat. No. 6,662,107 discloses a power saving mode which uses a separate clock circuit that is powered all the time. This clock circuit also provides corrections for temperature variations. The clock correction claimed in that patent (claim 19) is based upon the temperature and Doppler. U.S. Pat. No. 6,725,157 describes a procedure wherein the GPS receiver first operates outdoor and then maintains the lock when moved indoor. U.S. Pat. No. 6,757,610 discloses storing some parameters like clock Doppler, receiver velocity for use during the next acquisition or reacquisition.
Present day GPS receivers do not make extensive use of clock calibration. Therefore, there is a need for GPS receivers that use clock calibration to make faster reacquisition of GPS signals possible.